The present invention relates to safe arming and fuzing systems for the detonation of explosives and propellants. More particularly, it relates to electronic safe arming and fuzing systems.
Safe arming and fuzing (SAF) systems are extensively used in missiles. The function of the SAF is to maintain the missile warhead or rocket motor in a safe condition until the missile has separated from the launch vehicle some predetermined safe distance. At this point, the SAF arms the warhead. The SAF also fires the warhead on receipt of a trigger signal from either an external source such as a contact trigger or from a backup trigger which may be integral with the SAF device.
In missiles, the warhead is made with a secondary explosive as opposed to a primary explosive. Secondary explosives are characterized by having a very high energy output and very specific conditions for detonation. For example, it typically requires 10.sup.7 ergs delivered in less than 1 microsecond to detonate a secondary explosive. Primary explosives, on the other hand, are very sensitive to detonation, requiring about 1000 times less energy than a secondary explosive, but have a lower energy output. Primary explosives will detonate when subjected to a spark, flame, friction, a hot wire which causes a crystal of the primary explosive to reach its ignition temperature, or mechanical shock. In contrast, secondary explosives will detonate only when subjected to a high energy shock wave. The most common way of detonating a secondary explosive is with the shock wave generated by a primary explosive.
A small charge of primary explosive and some means for causing its controlled detonation is often referred to as a primer. A good example of a primer is a blasting cap consisting of a small charge of primary explosive such as lead azide molded around a small resistance wire. When an electric current is passed through the wire, the wire heats up very fast and causes the primary explosive to detonate. If the primer is located properly with respect to a secondary explosive, detonation of the former will cause detonation of the latter. A percussion primer operates in the same manner except that impact and friction energy are used to ignite the primary explosive rather than electrical energy.
Although the use of a primer is a cheap and effective means of detonating secondary explosives, it is very hazardous. Because primary explosives are sensitive to heat and shock they are prone to exploding prematurely. For example, the mere presence of radio frequency electromagnetic energy in the area may cause the resistance wire to heat up enough to detonate the primer. In spite of the considerable hazards associated with primers, they have been and continue to be the detonator around which nearly all SAF systems are built.
In attempting to avoid the hazards of a conventional primer, existing SAF systems are built in a very special way. Fundamental to existing SAF systems is the requirement that the primer be kept "out of line" with the secondary explosive until the warhead is armed. That is, prior to arming, the primary explosive is physically separated from the secondary explosive by a heavy mechanical barrier. The arming step consists of removing the mechanical barrier between two explosive types. This is done by the actual mechanical movement of the primary explosive from behind the barrier to a position that is in line with the secondary explosive. This requires an actual mechanical movement of parts within the SAF system and accordingly requires the application of mechanical energy. Typically this is achieved by loading a spring from energy gained through the acceleration of the missile. For example, the missile acceleration places a force on a movable heavy weight which in turn cocks a spring. Energy in the spring may then be used to move the primer in line with the secondary explosive at the appropriate time.
The process of determining that the missile has reached a safe distance from the launch vehicle is typically achieved by sensing that the missile has been launched with an accelerometer, which in turn initiates a classic mechanical clock movement. After the clock has counted down its predetermined period of time, it trips the spring and causes the primary explosive to move into alignment with the secondary explosive. The warhead is then in an armed condition and can be fired by a trigger signal which heats the resistance wire and sets off the primary explosive.
Although this approach to detonating missile warheads has proved successful and has been and is used extensively, it has several disadvantages. To start with, the required mechanical movement of the primary explosive requires that the SAF system be relatively large and heavy. The shelf life of the systems is relatively short due to the aging of lubricants. The systems require very sensitive detonators for a fast response time (in the neighborhood of 100 microseconds) and thus, are susceptible to instantaneous accidental closure of the trigger. If a missile should fail to explode after it is armed, or for some other reason, it may be desirable to disarm a missile once it is armed. To do this with a conventional SAF system is extremely dangerous since an armed conventional SAF system has a primary explosive in line with a secondary explosive so that it is subject to all of the hazards of a conventional primer. It takes mechanical energy to disarm the system. Furthermore, positive action must be taken by someone such as removing the detonator from the warhead or otherwise isolating the detonator from the next element in the explosive train. This is an exceedingly dangerous task.
A related problem with existing SAF systems is their inability to once armed be reset in flight to a disarmed condition based on external information. For example, if a missile to be used in close air support goes off course, it is desirable to disarm the missile so that it will not explode among friendly troops. Existing systems are incapable of doing this because they are usually locked in the arm position and require mechanical energy and frequently acceleration to achieve a disarm position.
It is therefore an object of this invention to provide a primarily electronic SAF system;
It is a further object of this invention to provide a SAF system that will disarm itself merely by the passage of time:
It is another object of this invention to provide a SAF system that will respond to off course signals and automatically disarm itself;
It is another object of this invention to provide an extremely small SAF system:
It is another object of this invention to provide an extremely reliable SAF system;
It is still another object of this invention to provide a SAF system that is relatively cheap to manufacture; and
Finally, it is an object of this invention to provide a SAF system that has a long shelf life.